A backplane bus is used in switching systems and the like to convey signals from circuitry on a first printed circuit board to circuitry on one or more other printed circuit boards served by the same bus. The bus itself is a printed conductive trace on an elongated board termed a backplane. This backplane has mounted on it a plurality of connectors into which the printed circuit boards are plugged. The conductors of the backplane bus connect to pins of the connectors so that the circuitry mounted on the different printed circuit boards plugged into the connectors can intercommunicate via the backplane bus. A plurality of interconnected printed circuit boards and the backplane bus serving these boards constitute what is termed herein as "a bus group".
In accordance with one prior art arrangement, all connectors mounted on a backplane board are often connected together electrically by the board's backplane bus at the time of design and manufacture of the backplanes. This is a disadvantage because only a single bus group can be served by such a single backplane bus. This prevents a backplane board from having the capability of serving more than one bus group or serving a varying number of groups of printed circuit boards in the event the needs of the system user change after installation.
Strategies have been devised to deal with this problem. One strategy is, after manufacture, to physically cut the printed conductive trace constituting the backplane bus. The backplane bus is cut between two adjacent backplane connectors. This way, circuit boards plugged into connectors on one side of the cut make up one bus group, and the boards plugged into connectors on the other side of the cut make up another bus group. This strategy has the disadvantages that the backplane cannot be reconfigured easily back to its original design. Another disadvantage is that if an error is made in the cutting operation, the backplane must be discarded and replaced since it cannot easily be physically restored. The process of planning the cut and cutting the bus is an expensive manual operation. Also, if the backplane bus is a conductive trace in the middle of a multi-layer board, it may be impossible to cut.
Another strategy for enabling a single backplane bus to serve a different number of bus groups is to manufacture a number of different models of the backplane in which there are open bus connections between adjacent backplane connectors as required to define the number of board groups to be served. This way, circuit boards plugged into connectors on one side of an open connection make up one bus group, and boards plugged into connectors on the other side of the open circuit make up another bus group. Each different backplane model has the open circuit between different backplane connectors as required. One model, for instance, may have an open circuit between the third and fourth connectors of the backplane. Another model may have an open circuit between the fifth and sixth connectors. This strategy has the disadvantage of increasing the number of backplane models that must be maintained in inventory.